Recent developments of projection display devices that can enlarge and project information from a display, such as a light valve, a computer image or a television image have been remarkable. A zoom lens, which is capable of changing the size of a projected image, is frequently used as the projection lens in projection display devices. More recently, a zoom lens having a large ratio of change, i.e., a zoom lens having a large zoom ratio, has been increasingly desired.
A so-called lens shift projection function in which an image greatly shifts in position relative to the projection display device during projection and a function in which a large projected image can be seen in a position close to a projection display device have also been required, which, in turn, requires widening the projection angle of the zoom lens. Additionally, such a zoom lens with a widened projection angle that provides a brighter and sharper image has been desired.
Zoom lenses in which three or more lens groups are moved during zooming and that are telecentric, or at least nearly telecentric, on the short conjugate length side are known, and are, for example, described in Japanese Patent No. 3,362,613 and Japanese Laid-Open Patent Application Nos. 2001-337275, 2001-350096, 2002-131641, 2003-015036, 2003-015037, and 2003-015038. The zoom lenses described in Japanese Patent No. 3,362,613 and Japanese Laid-Open Patent Application Nos. 2001-337275 and 2001-350096 have a zoom ratio of 1.5 or more, but a zoom lens that includes a larger field angle is presently desired. On the other hand, the zoom lenses of Japanese Laid-Open Patent Application Nos. 2002-131641, 2003-015036, 2003-015037, and 2003-015038 are wide-angle zoom lenses that include field angles of sixty degrees or more, but their zoom ratios are smaller. Additionally, many of the prior art zoom lenses have f-numbers of 1.8 or more, and a brighter lens is presently desired for marketing.
As described above, zoom lenses are available that achieve one of the required wide-angle projection or the desired zoom ratio, but there are no zoom lenses that achieve both or that further satisfy the brightness requirements. The difficulty of simultaneously satisfying the wide-angle, zoom ratio, and brightness requirements is indicated by the zoom lenses presently used as projection lenses of projection display devices.